1. Field
The present invention relates generally to wireless communications, and more specifically to limiting electromagnetic radiation emissions in wireless communications devices.
2. Background
Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources such as bandwidth and transmit power. Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3GPP Long Term Evolution (LTE) systems, 3GPP2 Ultra Mobile Broadband (UMB) systems, LTE Advanced, WirelessMAN-Advanced, and orthogonal frequency division multiple access (OFDMA) systems.
Increasing the amount of power transmitted in a system can increase the data throughput and transmission range of the device, enabling rich multimedia applications in wireless devices such as smartphones, WAN-WLAN gateways (e.g. MiFi) and tablets. Increased transmission range enables these applications to be used even at locations far away from cellular base stations or access points. Thus, increasing the maximum allowed transmit power of wireless devices is beneficial for user experience.
While increasing transmit power improves multiple-access communication system functionality and user experience, it also increases Radio Frequency (RF) radiation from the wireless device. Users and regulatory agencies are concerned about the deleterious effects of increased RF radiation on user health. Studies have been performed on the effects of RF radiation in relation to cancer, reproductive health and other illnesses. Regulatory agencies such as the FCC have imposed limits on RF radiation from mobile devices specified in terms of Specific Absorption Rate (SAR). SAR is a measure of the rate at which energy is absorbed by the body when exposed to an RF electromagnetic field. SAR is defined as the power absorbed per mass of tissue in units of watts per kilogram (W/kg). SAR is usually averaged either over the whole body, or over a small sample mass of tissue between 1 and 10 grams. The SAR value is then the maximum level measured in the affected body part over the stated mass. SAR is a function of the design of the wireless device, its placement with respect to the user's body, and the level of transmit power. Wireless devices currently limit their transmit power to a fixed value. For example, some devices use a limit of 24 dBm to satisfy SAR limits. Additionally, wireless device manufacturers also advise users to place the devices sufficiently far away from the body, such as 1 inch away from the head, in order to limit its SAR.
However, the traditional use of a fixed transmit power limit often results in the chosen transmit power limit being too high or too low for a particular individual user. The fixed power limit is typically chosen by a device manufacturer based on a statistical sampling of user physical characteristics and device placement near the body. The limit imposed may be too high for individual users including infants and children, pregnant women, elderly or persons with sensitive physical characteristics. A fixed transmit power limit may also be too high for habitual usage patterns such as transmitting background data at maximum power while the device is in close proximity to vital organs. For example, when a wireless device is kept in a person's pocket. Unlike voice-only mobile phones where users knew their phone was transmitting only while engaged in a call, users of wireless devices in later generation multiple-access communication systems may be neither aware of, nor warned by, the device of ongoing transmission activity due to high power background uploads, data syncing or other applications. Furthermore, unaware bystanders may be subject to “second-hand radiation” from devices owned by others.
Conversely, the fixed transmit power limit may be too low for an individual user. While setting a low transmit power limit ensures that the device will produce acceptable SAR for a wide range of usage scenarios, the limit may be too low for individual usage behavior. For example, a SAR limit set assuming a smartphone is placed next to the head is too low for a user who always uses a headset for voice calls in order to place his smartphone away from his body.
There is therefore a need in the art for a method for dynamic individual control of transmit power in wireless devices to limit electromagnetic RF radiation.